Thanks for checking out my game, I'm glad you had enjoyed it! Also I'd be happy to share some of the experience I got making this project;
I love playing rhythm games (Project Diva, Unbeatable and Osu! are some favorites :D) so I wanted to make a game that I could see myself playing. Without getting too engine-specific, I think one of the hardest unforseen challenges was learning what makes a good rhythm game chart (the sequence of notes/commands the player has to press representing the song).
Rhythmically speaking, pop music typically isn't very dynamic. If we chart it so the player is literally "tapping to the beat", all the player would be doing is tapping out 1-2-3-4, 1-2-3-4 which would be quite boring haha. To make it more interesting to play we can have the player jump between different parts of the song. At first they could be tapping along to the drumbeats, then when the vocals kick in they can switch to the vocal melody, then on the instrumental break they can switch to the backing melody. As a game designer there are a lot decisions you can make about how you want to represent the song you are charting. The way I've come to understand it, a good chart tows a line of keeping the player on their toes, but also stays predictable enough that a skilled player should be able to score decently on their first try. It took a good bit of trial and error for me to find what I wanted the player to be doing at each section of the song, and even then there's still a lot I want to tweak in the current version.
Hopefully this is helpful for you, and if you have any more questions feel free to dm me on discord @andysakano, I'd be happy to answer!